As the electronic techniques have been employed more widely in automobiles, various kinds of semiconductor apparatuses are used. It is required to control the main current of the semiconductor apparatuses precisely irrespective of whether the value thereof is small or large.
For measuring the main current, a small sensing MOSFET and a power MOSFET are mounted on a same semiconductor substrate and the main current value is measured from the value of the current that flows through the sensing MOSFET. In detail, the current that flows through the sensing MOSFET is made to flow through a sensing resistance and the main current value is measured from the voltage drop cause across the sensing resistance.
FIG. 22 is a top plan view of conventional power MOSFET 700 that includes a sensing MOSFET. FIG. 23 is a cross sectional view of conventional power MOSFET 700.
Referring now to FIGS. 22 and 23, conventional power MOSFET 700 includes chip 96, on which main gate electrode pad 8 is formed. Main gate electrode pad 8 is connected to source electrode 80 of main MOSFET 91, source electrode 81 of sensing MOSFET 92 and gate electrode 6 of main MOSFET 91 via gate wiring 7.
The ratio of the current IO that flows through main MOSFET 91 and the current IS that flows through sensing MOSFET 92 is the sensing ratio (IO/IS).
The design value of the sensing ratio is determined by the ratio of the total area of n+ source layer 5 in main MOSFET 91 and the total area of n+ source layer 5 in sensing MOSFET 92.
FIG. 24 is a circuit diagram of conventional power MOSFET 700 that includes a sensing MOSFET.
Referring now to FIG. 24, power MOSFET 700 includes main MOSFET 91 that makes a main current flow and sensing MOSFET 92 that detects the main current made to flow by main MOSFET 91. Drain electrode 13 of main MOSFET 91 and sensing MOSFET 92 is connected to high potential side 75 of a power supply via drain terminal 18. Source terminal 85 of main MOSFET 91 is connected to load 71 such as a resistive load and an inductive load. Load 71 is connected to ground 76 of the power supply. Source terminal 86 of sensing MOSFET 92 is connected to sensing resistance 72. Sensing resistance 72 is connected to ground 76 of the power supply. The gates of main MOSFET 91 and sensing MOSFET 92 are connected to gate wiring 7. Gate wiring 7 is connected to control circuit 74 via gate electrode pad 8 and main gate terminal 14.
The sensing current IS that flows through sensing MOSFET 92 flows to ground 76 through sensing resistance 72. The voltage drop caused across sensing resistance 72 by the sensing current IS is fed to detector circuit 73. By the signal fed from detector circuit 73, the main current IO of power MOSFET 700 is controlled through control circuit 74.
In FIG. 23, n+ drain layer 1, n− drift layer 2, p− well layer 3, trench 4, interlayer insulator film 9, and unit cell 43 are shown. In FIG. 22, casing 97 is shown.
Now the operations of power MOSFET 700 will be described below.
The main current IO flows from the power supply to main MOSFET 91 and to load 71. As the main current IO flows to load 71, the sensing current IS flows through sensing MOSFET 92 and through sensing resistance 72.
If the total area of n+ source region 5 in sensing MOSFET 92 is set to be 1/10000 times as wide as the total area of n+ source region 5 in main MOSFET 91, the sensing current IS will be 1/10000 times as large as the main current IO. Therefore, the sensing ratio (IO/IS) will be 10000. By feeding the voltage drop caused across sensing resistance 72 by the sensing current IS to detector circuit 73, the main current value IO of main MOSFET 91 is measured. The ratio between the total area of n+ source region 5 in main MOSFET 91 and the total area of n+ source region 5 in sensing MOSFET 92 provides the design value of the sensing ratio.
FIG. 25 illustrates the relation between the sensing ratio and the main current of the main MOSFET.
As described above, the sensing ratio is the ratio (IO/IS) between the main current IO that flows through main MOSFET 91 and the sensing current IS that flows through sensing MOSFET 92. Sensing ratio variations increase as the main current IO becomes smaller as described in FIG. 25, unstabilizing the sensing ratio. In other words, the sensing ratio deviates more from a certain value, as the main current IO becomes smaller.
When the main current IO is around 10 A or higher in FIG. 25, the sensing ratio is stable, exhibiting only small deviations. When the main current IO is small, e.g. several A, large deviations are caused in the sensing ratio, the sensing ratio does not exhibit a certain value, and the sensing ratio is unstable. If the sensing ratio is unstable, it will be impossible to determine the current flowing to load 71 precisely. To obviate this problem, a power MOSFET, the sensing ratio thereof being small, is selected according to the prior art in the case, in which the current level of load 71 is low.
The following Patent Document 1 discloses a semiconductor apparatus that includes a power MOS transistor and a current detecting transistor that detects the current of the power MOS transistor and generates a detection signal fed to an external control circuit. The semiconductor apparatus disclosed in the Patent Document 1 includes also a protector circuit that causes a lowering of the gate voltage of the power MOS transistor, when a current higher than a predetermined value flows through the power MOS transistor. The semiconductor apparatus disclosed in the Patent Document 1 includes the power MOS transistor, the current detecting transistor, and the devices constituting the protector circuit mounted on the same semiconductor chip. The Patent Document 1 discloses also a control system employing the semiconductor apparatus.
The following Patent Document 2 discloses a semiconductor apparatus that includes a gate electrode pad and a driving region, divided into a same number of constituent gate electrode pads and the same number of constituent driving regions, respectively. The constituent driving regions used are changed depending on the driving conditions to reduce the losses.